Saturday, December 4, 2010

Recently there has been quite the flurry of tweets about OOP (objected oriented programming) and DoD (data oriented design). If you're unfamiliar with DoD, here's a nice presentation. If you're unfamiliar with OOP, I'd like to know what cave you've been living in for the last few decades.

DoD has caught on with game programmers because it puts a name to something anyone who has spent time optimizing a game already knew -- your data access patterns have a much bigger impact on your performance than the actual code you execute. I remember many an optimization session on Stranglehold where a reduction in L2 cache misses led to a perfectly correlated reduction in execution time.

DoD goes farther in that it presents a set of guidelines for writing code up front that will run under the reality of the memory-processor speed gap. This does set it apart from a simple optimization technique as it is something you can use before the fact, rather than after the fact. Follow these guidelines, your program will perform better.

Dino Dini argues that this is nothing new, that game programmers have been doing this for decades. He's right. The underlying concepts are not that new, but giving it a name and a simple package of guidelines is new. This has value, I think, because it helps educate programmers about these concepts. I am not discounting anyone's effort in this area, because I think a lot of programmers need to learn these concepts.

That said, I don't think DoD approaches what one would call a programming paradigm. The consensus definition of programming paradigm is a "fundamental style of programming." It certainly is a style of programming, but I don't think it is fundamental.

While I put on my flame retardant, let me explain what I mean. Structured/procedural and OOP are two programming paradigms that historically grew out of the need to manage software complexity. These are paradigms in which you could organize an entire code base. They contain methods for abstraction, and layered design.

DoD says nothing about code complexity. It does not describe how to organize your entire code base. No matter what happens with the processor-memory gap, code complexity is a huge problem for any large project. DoD offers no tools for managing this complexity.

I can imagine a code base completely organized around the structured paradigm (and many exist). The same with OOP. Many real world code bases mix a little bit of both paradigms -- platform APIs tend to be structured, application architecture these days tends to be OOP.

I can see how DoD fits into either of these paradigms. I don't know what a code base completely organized around DoD would look like. I don't think that's even a question that makes sense, as it is not tackling the same set of problems.

This is fine, and does not take away from DoD at all. In fact, I think it frees us to discuss the realities of writing software for today's hardware without having to waste time arguing about OOP vs DoD. They are apples and oranges.